ADP-glucose pyrophosphorylase (hereafter, referred to as “AGPase”) is a heterotetramer comprising large subunits and small subunits encoded by different genes, and it is an enzyme associated with the starch synthesis pathway. Corn with lowered AGPase activity has heretofore been used for sweet corn breeding and its high sucrose content in seeds (albumens) has been known (Non-Patent Document 1). Also, Patent Document 1 discloses a method for increasing the seed yield of corn by inducing AGPase to express in a stage- or site-specific manner.
Non-Patent Document 2 discloses that inhibition of AGPaseB gene expression via RNAi in potato results in decreased starch content in the tuberous root and in increased sucrose content to a level approximately 10 times greater than that of a wild-type plant. Further, Patent Document 2 discloses that inhibition of AGPase activity in garden pea results in increased sucrose content in beans. Regarding garden pea, Non-Patent Document 3 also discloses that decreased starch content and increased sucrose content were observed in mutant garden pea germs exhibiting approximately 3% to 5% AGPase activity.
Further, Non-Patent Documents 4 and 5 each disclose the correlation between decreased starch content and lack of AGPase activity in Arabidopsis thaliana. 
Furthermore, Non-Patent Document 6 discloses the floury 2 mutant that affects expression of a plurality of enzymes associated with starch biosynthesis, such as DBE or AGPase, in rice. Regarding rice-derived AGPase, four types of genes (i.e., AGPL1, AGPL2, AGPL3, and AGPL4 genes) are known as genes encoding large subunits, and two types of genes (i.e., AGPS1 and AGPS2 genes) and two types of AGPS2 gene transcription products (i.e., AGPS2a and AGPS2b) are known as genes encoding small subunits. Non-Patent Document 7 discloses that the seed amount is increased in rice into which E. coli-derived mutant AGPase (Pi-insensitive or constitutively active) has been introduced.
As disclosed in Patent Documents 3 and 4, regarding modification of AGPase activity, it is known that introduction of site-directed mutation into the AGPS gene encoding the small subunit results in attenuated sensitivity to inorganic phosphate (Pi) and increased seed weight or amount. Thus, the AGPase activity of a plant was known to influence traits such as seed weight or amount.